Oxygen deprivation is a major cause of neurodevelopmental impairments in preterm infants. Cortical gray matter loss and cognitive disability improve over time in some children, while others remain cognitively impaired. The mechanisms undertying this variable recovery are unknown. Our mouse model of chronic sublethal hypoxia reproduces the initial brain atrophy as well as the subsequent recovery in brain structure. In the recovery period following hypoxia, we observed increased proliferation of neural stem and progenitor cells (NSC/NPCs) as well as generation of new cortical neurons from these precursors. Despite this, longterm deficits in working memory persist in the hypoxia-reared animals and significant decreases in the number of GABA interneurons remain in the cerebral cortex. The goals of Project 1 are to investigate the mechanisms for the differential response of excitatory and inhibitory neurons to hypoxia and enhance anatomical and functional recovery following the insult. In Aim 1, we will study whether the long-term interneuron deficiency after hypoxia is due to a loss of cells or to a deficient maturation of GABAergic inhibitory properties by immunocytochemical and electrophysiological analyses in Gadl'^''^^* mice. In Aim 2, we will examine whether environmental enrichment promotes the generafion of new excitatory and inhibitory neurons and glial cells from GFAP+ NSC/NPCs or their survival and synapfic integrafion into the circuitry. These studies will use genetic fate mapping in GFAP-CreERT2 mice and, in collaboration with Projects 3 and 4, electrophysiology and electron microscopy. In Aim 3 we will examine whether BDNF signaling is required for the beneficial effects of environmental enrichment on neuronal survival by comparing wild type and TrkB-null cell lineages in mice with inducible delefion of TrkB receptors in GFAP+ cells. In Aim 4 we will test whether exogenously increased FGF signaling, together with environmental enrichment, improves motor and cognitive funcfion by enhancing inhibitory interneuron development after hypoxic insult, including the maturation of their synaptic connecfions probed by ultrastructural studies. Identifying the crucial cell populations and molecular ingredients that are required for appropriate reconstitufion of neurons and glial cells in cortex will permit us to enhance these growth-promoting responses in critically ill children.